The present invention relates to a method of and apparatus for testing whether a natural diamond has had a layer of synthetic diamond deposited thereon. This is of particular importance in testing whether the diamond is wholly natural or whether any part of it comprises CVD diamond material and also in locating such material if present.
Synthetic diamond material may be deposited on an uncut or part processed natural diamond which is then worked, for example, into a round brilliant cut. Alternatively, the synthetic diamond material coating may be deposited onto a fully fashioned brilliant stone after working of the stone. The thickness of the synthetic diamond material layer may be very thin (it could be in the range from 5 microns to 10 microns) but the present invention may also be used to detect thicker layers.
The value of a diamond is in part dependent upon its weight. Accordingly, synthetic diamond material may be deposited onto natural gem diamonds, before or after cutting of the diamond, to increase the weight of the finished product.
However, the value of a diamond also resides in its qualities of authenticity and uniqueness and in the fact that it is an entirely natural (i.e. mined) product. Thus, a diamond that has not been enlarged by deposition of synthetic diamond material has a value over a diamond which has.
Over the years, a number of methods of synthesising diamond material have been developed. One of these methods is the chemical vapour deposition (CVD) technique, which is a low pressure technique involving deposition of synthetic diamond (referred to as CVD diamond material in this specification) onto a substrate from a gas. CVD is the most likely way in which synthetic diamond will be deposited on a diamond, although alternative techniques such as physical vapour deposition have been proposed. A diamond artificially enlarged by deposition of CVD or similar diamond material is referred to in this specification as a "CVD/natural diamond doublet".
CVD diamond material may be deposited on a non-diamond or diamond substrate. In the latter case, the CVD diamond material can replicate the structure of the diamond substrate (referred to as "homoepitaxial growth"). The CVD/natural diamond doublet produced can be identical in appearance, density and other common physical properties to an entirely natural stone and there may be a problem in identifying such a CVD/natural diamond doublet.
It is an object of the present invention to provide a method of and apparatus for testing whether a diamond has had a layer of synthetic diamond deposited thereon.
It is desired that the apparatus should be simple and may be put into operation by a person with relatively little training. The method and apparatus should be capable of being operated reliably and consistently by a practised jeweler who has no training in laboratory gemological analysis. The method and apparatus should be suitable for screening large numbers of stones, one at a time, and should be suitable for automation.
British patent application No 9404309.8 discloses a method of determining whether a diamond has had a layer of synthetic diamond deposited thereon in which the diamond is caused to luminesce with electrons or high energy ultraviolet radiation and the resulting pattern of luminescence is observed to detect zones of superficial synthetic diamond. Preferably, the whole diamond is irradiated and the pattern observed by eye through magnifying means or on a screen via a CCD camera.
The present invention provides a method of testing whether a diamond has had a layer of synthetic diamond deposited thereon, comprising observing a plurality of zones of the surface of the diamond, each zone being observed by irradiating the zone with high energy radiation to excite or stimulate emission of luminescence and assessing the intensity of the luminescence.
The present invention further provides an apparatus for testing whether a diamond has a layer of synthetic diamond deposited thereon, comprising a mounting means, a support for a diamond, movably mounted on the mounting means, means for irradiating a diamond supported in the support with high energy radiation to excite or stimulate emission of luminescence, and means for providing a signal dependent upon the intensity of luminescence produced when a diamond mounted on the support is irradiated. Means for driving the support with respect to the mounting means may be provided. The mounting means may be fixed with respect to the irradiating means.
The invention further provides an apparatus for testing whether a diamond has had a layer of synthetic diamond deposited thereon, comprising an integrating enclosure having a support for a diamond, means for irradiating a zone of a diamond mounted on the support, and means for giving a signal dependent upon the flux intensity of luminescence in the integrating enclosure, produced when a diamond in the integrating enclosure is irradiated. The support for the diamond may be movable with respect to the integrating enclosure and may be driven by drive means.
The inventors have discovered that seeking substantial differences in the luminescence of different zones of a diamond provides a particularly simple way to locate superficial layers of synthetic diamond material. No imaging or visual interpretation of a complex image by an operator, as in British patent application number 9404309.8, is required.
By luminescence is meant emitted radiation of a wavelength generally different to the irradiating radiation which causes it.
The luminescence intensity is preferably measured. Preferably, a signal dependent upon the intensity of luminescence from each zone is produced. Alternatively, the surface of the diamond may be scanned by a beam of irradiating radiation, any significant change in intensity of luminescence between one zone and the next being detected.
The diamond may be irradiated with ultraviolet radiation of suitable wavelength. Substantially all natural diamonds will luminesce if irradiated with radiation of wavelength less than 225 nm. It is accordingly preferable to use radiation of wavelength less than or approximately equal to 225 nm. The irradiating radiation may be substantially monochromatic or it may comprise a range or a set of wavelengths.
It is preferred that preponderantly only the surface region of the diamond is irradiated and caused to luminesce. This is because layers of synthetic diamond material may be relatively thin. If the irradiating radiation penetrates to a depth significantly greater than the thickness of the thin layer of synthetic diamond material, luminescence could be produced from underlying natural diamond material which would confuse or swamp out the luminescence from the synthetic diamond layer.
For this reason also, it is preferred that the diamond is irradiated with radiation of wavelength less than or approximately equal to 225 nm which is very strongly absorbed by all types of diamond. This is described in more detail in British patent application number 9404309.8.
The irradiating radiation may include radiation of wavelengths greater than 225 nm. Certain radiation bands of wavelength greater than 225 nm have different absorption characteristics in different types of diamond. Accordingly, such radiation could penetrate the layer theoretically being studied and cause luminescence in other areas of the diamond, which could confuse the results. Irradiating radiation of wavelength much greater than 225 nm may be confused with luminescing radiation. It is desirable that radiation of wavelength greater than 225 nm should be sufficiently low in intensity that luminescence from parts of the diamond apart from the zone of interest does not swamp out or reduce the contrast in observations of luminescence. Preferably at least 50% of the irradiation energy is at wavelengths less than 225 nm. Preferably, however, radiation of wavelengths greater than 225 nm should be substantially excluded by a suitable filter.
The diamond may alternatively be irradiated with a beam of electrons of suitable energy, but the apparatus would then be complicated.
The irradiating radiation must be of intensity sufficient to generate observable luminescence.
The irradiating radiation may be generated by any suitable means, for example a laser or other source. The irradiating radiation may be directed onto the gemstone by any suitable means. However, the attenuation of short wavelength ultraviolet radiation by normal optics is high and it is preferred to use optical equipment which has a high transmissivity at short ultraviolet wavelengths.
Radiation of wavelengths shorter than 180 nm is attenuated by normal UV optics and by oxygen in air and is effectively filtered out by the apparatus.
Preferably the radiation is focused onto the diamond. More preferably, the radiation is focused onto an area of the diamond which is smaller than the total presented surface area of the diamond. Most preferably the radiation is focused to a small spot and scanned over the surface of the diamond.
As set out in more detail in GB 9404309.8, radiation of wavelength less than 225 nm is absorbed predominantly in the surface region of the diamond. This is of assistance in the present invention in that luminescence observed when a given zone is irradiated will be predominantly dependent upon the composition of the surface of the zone irradiated.
The luminescence bands observed for various types of diamond (natural or synthetic) fall within a wide range of wavelengths, generally in the visible part of the spectrum. A signal dependent upon intensity of luminescence falling in a relatively narrow band or a relatively wide band may be given. In the latter case, it is preferable to provide a cut-off filter to exclude the irradiating radiation.
A synthetic diamond layer deposited upon a natural diamond may be identifiable if the luminescence thereof is a different colour to the luminescence of the natural part of the diamond or, more importantly, of a different intensity to the luminescence of the natural part of the diamond. Accordingly, when the plurality of zones are tested, significant differences (for example, the lower signal being of the order of 80%, preferably 50%, or less of the higher), in the intensity of luminescence produced by different zones of the diamond will suggest a CVD/natural diamond doublet. It is possible that differences in luminescence intensity do not originate in a layer of synthetic diamond. The present invention provides a useful guide. However, further testing may be beneficial.
It may be sufficient only to test a few zones (maybe only two) in order to detect a difference in the luminescence in different zones. Preferably, however, a large number of zones are observed.
The intensity of radiation may, in the method of the invention, be assessed by eye. In this case, means should be provided for exluding the hazardous ultraviolet radiation from the observer. If the luminescence is assessed by eye, it is not necessary to form an image of the zone irradiated if the irradiating radiation can be confined to the zone of interest and irradiation of other zones avoided. In that case, the luminescence, rather than the diamond will be observed in effect.
Preferably, the observed radiation comprises no irradiating radiation. A small amount of irradiating radiation may be tolerated in the observed radiation if it does not swamp out luminescence.
The luminesced radiation may be detected by any suitable means. For example, a beam splitter may be placed in the path of the irradiating radiation, being configured to direct luminesced radiation from the diamond to a detector. A filter for filtering out irradiating radiation may be provided for the detector.
Alternatively, the diamond may be placed in an integrating enclosure and a zone of the diamond irradiated with irradiating radiation. The integrating enclosure is provided with a detector for giving a signal dependent upon the intensity of luminescence in the integrating enclosure produced when the given zone is irradiated. The detector may include a filter for filtering out irradiating radiation.
Preferably, the integrating enclosure comprises an integrating sphere.
If an integrating enclosure is used, the zone of the diamond of interest must be irradiated and substantially no other zones.
The diamond may be irradiated using a beam of confined dimensions which may be produced, for example, by an aperture between the diamond and the radiation source.
Preferably, a single zone of the diamond is irradiated at any one time and a plurality of such zones are irradiated sequentially. However, a plurality of different zones of the diamond may be independently irradiated simultaneously and signals dependent upon the intensity of luminescence produced by each respective zone provided, in succession or simultaneously, the observations being subsequently compared.
The diamond may be placed with the zone of interest in contact with the aperture, to reduce the inclusion of light from other parts of the diamond. This arrangement is particularly suitable if a beam splitter is provided in the irradiation path for passing luminescence to a detector.
The confined beam may be of variable dimension or of fixed dimension. It may correspond in size to a facet of a worked diamond or to a part of a facet. Preferably, the confined beam is smaller than the maximum dimension of the diamond, or is adjustable in size to allow this. The aperture may be of size 1-15 mm across, preferably 5-10 mm. An iris aperture may be provided, adjustable in size for best results.
More preferably, the beam may be focussed to a small spot of size 1 micron-1 mm across, preferably 5-100 microns, and preferably scanned across the diamond.
Radiation emanating from the diamond may be passed to the detector through a filter. Preferably, the filter is a cut-off filter for filtering out the irradiating radiation. A further filter may be provided for passing selected luminescence bands. For example, a number of interchangeable filters could be used, each passing light of a different wavelength.
The beam is preferably scanned (ie moved continuously or semi continuously) over the surface of the diamond. Means for scanning the beam may be provided in the form of means for moving the beam with respect to the diamond. For example, the diamond may be rotated about an axis not coincident with the beam of radiation. Preferably, the axis is normal to the beam of radiation. Means may be provided for moving the diamond linearly with respect to the beam of radiation, for example in two directions normal to the beam of radiation.
Means may be provided for giving a signal if the intensity of radiation emitted by the diamond changes by an amount exceeding a predetermined value. This is particularly useful if the diamond is scanned continuously. It allows changes in surface composition to be readily identified. For example, the signal giving means may comprise means for giving a signal dependent upon intensity of radiation and signal generating means for giving a change signal if the intensity of radiation changes by a given amount. For example, the change signal may be given if the intensity of radiation measured changes by 5%, preferably by more than 10%, preferably by greater than 20%.
The signal generating means may comprise a timer so that a change signal is only given if the intensity of radiation changes by a predetermined amount within a predetermined period of time. Means may be provided for altering the period of time and/or the amount by which the signal must change before a signal is given.
In one embodiment of the invention, the diamond is placed in a rotatable mount and rotated continuously, whilst the intensity of luminescence is measured. A noisy or modulated DC signal (variations in intensity of luminescence being caused by naturally occurring slight local differences in diamond composition, and internal reflection and refraction) followed by a much broader pulse of higher or lower intensity will suggest a CVD/natural diamond doublet.
Preferably, the diamond is rotated a plurality of times in order to give a plurality of readings which may be combined statistically to give a statistically improved reading.
The invention is preferably used with fluorescence--that is, luminescence produced effectively instantaneously by a zone of a diamond when it is irradiated with an electron beam or high energy ultraviolet radiation.
The apparatus of the invention is preferably confined in a light-tight box. This is to exclude radiation from external sources from reaching the detector and to prevent the potentially harmful high energy ultraviolet radiation escaping and causing damage to skin and eyes.